Looking at the implementation of Xor, Nand, Nor, and Not classes, it
appears that the aim is to maintain formulas in negation normal form (
http://en.wikipedia.org/wiki/Negation_normal_form ). This would imply
(excuse the pun ;)) that the Implies class should do the same.
On Fri, Mar 19, 2010 at 4:52 PM, Vinzent Steinberg <
vinzent.steinb...@googlemail.com> wrote:
> What is the motivation of this? It would be nice if you could add this
> to the commit message.
> 'Implies(x, y)' is clearly less readable than 'Or(Not(x), y)', imho.
>
> Fabian, what is your opinion about this?
>
> Vinzent
>
> 2010/3/17 Christian Muise <christian.mu...@gmail.com>:
> > ---
> > doc/src/modules/logic.txt | 7 ++++---
> > sympy/logic/boolalg.py | 12 +++++++++++-
> > sympy/logic/tests/test_boolalg.py | 6 ++++--
> > 3 files changed, 19 insertions(+), 6 deletions(-)
> >
> > diff --git a/doc/src/modules/logic.txt b/doc/src/modules/logic.txt
> > index 2487d57..e466d5a 100644
> > --- a/doc/src/modules/logic.txt
> > +++ b/doc/src/modules/logic.txt
> > @@ -24,12 +24,13 @@ You can build boolean expressions with the standard
> python operators & (And),
> > >>> ~x
> > Not(x)
> >
> > -You can also form implications with >>, <<, and class Equivalent:
> > +You can also form implications with >>, <<, and class Equivalent, which
> are
> > +compiled away into their logical equivalents:
> >
> > >>> x >> y
> > - Implies(x, y)
> > + Or(Not(x), y)
> > >>> x << y
> > - Implies(y, x)
> > + Or(Not(y), x)
> >
> > Like most types in SymPy, Boolean expressions inherit from
> sympy.core.Basic:
> >
> > diff --git a/sympy/logic/boolalg.py b/sympy/logic/boolalg.py
> > index cd979bd..0a45e42 100755
> > --- a/sympy/logic/boolalg.py
> > +++ b/sympy/logic/boolalg.py
> > @@ -104,7 +104,17 @@ def eval(cls, *args):
> > return A
> >
> > class Implies(BooleanFunction):
> > - pass
> > + """Logical implication.
> > + A implies B is equivalent to !A v B
> > + """
> > + @classmethod
> > + def eval(cls, *args):
> > + if len(args) < 2:
> > + raise ValueError, "Only %d operand(s) used for an Implies
> (pairs are required): %s" % (len(args), str(args))
> > + elif len(args) > 2:
> > + return map(cls, args)
> > + else:
> > + return Or(Not(args[0]), args[1])
> >
> > class Equivalent(BooleanFunction):
> > """Equivalence relation.
> > diff --git a/sympy/logic/tests/test_boolalg.py
> b/sympy/logic/tests/test_boolalg.py
> > index 3102f06..e28f063 100755
> > --- a/sympy/logic/tests/test_boolalg.py
> > +++ b/sympy/logic/tests/test_boolalg.py
> > @@ -97,8 +97,10 @@ def test_Nor():
> >
> > def test_Implies():
> > A, B, C = symbols('ABC')
> > - Implies(True, True) == True
> > - Implies(False, False) == False
> > + assert Implies(True, True) == True
> > + assert Implies(True, False) == False
> > + assert Implies(False, True) == True
> > + assert Implies(False, False) == True
> > assert A >> B == B << A
> >
> > def test_Equivalent():
> > --
> > 1.6.3.3
>
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